1. Field of the Invention
The present invention relates generally to apparatuses and methods for delivering diagnostic and therapeutic agents into a living organism and, in particular, to apparatuses and methods for delivering such diagnostic and therapeutic agents intravascularly. The invention includes embodiments of various shapes and structures that are used to infuse therapeutic agents or deliver such agents intravascularly in the form of a soluble coating, and other embodiments that circulate or contain a preloaded charge of radioactive material for intravascular radiotherapy.
2. Description of the Related Art
Atherosclerosis is a cardiovascular disease in which deposits of plaques (atheromas) containing cholesterol, lipid material, foam cells, lipophages and proliferating smooth muscle cells are within the intima and media of large to small diameter arteries such as the aorta and the iliac, femoral, coronary and cerebral arteries. The resultant stenosis causes a reduction in blood flow.
Attempts to treat atherosclerosis have included bypass surgery wherein the diseased vascular segments are augmented by prosthetic or natural grafts. This procedure requires general anesthesia and a substantial healing period after surgery and, thus, is generally limited to cases of severe coronary artery disease.
Other approaches for the recanalization of stenotic vessels include percutaneous transluminal coronary angioplasty (PTCA), atherectomy, stenting and newer modalities of cardiovascular intervention, including laser angioplasty. The primary drawbacks of these procedures has been the appearance of restenosis at or near the site of the original stenosis in the blood vessel that requires a secondary angioplasty procedure or a bypass surgery. Another occurrence that reduces the success of a typical angioplasty procedure is that frequently the stenotic plaque or intima of the blood vessel or both are dissected during the angioplasty procedure by the inflation of the balloon. Upon the deflation of the balloon, a section of the dissected lining (commonly termed "flap") will collapse into the bloodstream, thereby closing or significantly reducing the blood flow through the vessel. In these instances, emergency bypass surgery is often required to avoid a myocardial infarct distal to the blockage.
In recent years, various devices and methods (other than bypass surgery) for prevention of restonosis and for repairing damaged blood vessels have become known. These methods typically use an expandable cage or region (commonly termed "stunt") on the distal end of a catheter designed to hold a detached lining against an arterial wall for extended periods to facilitate the reattachment thereof. Some stents are designed for permanent implantation inside the blood vessel, and others are designed for temporary use inside the vessel.
Typically, the expandable region of the prior art stents is formed by a braided wire or balloon attached to the distal end of the catheter body. Such designs are difficult and expensive to manufacture, and create reliability concerns due to the existence of high stress points located at the connection of the braided wire region with the catheter body and at the connections between the intermingled wire strands.
Alternatively, or in addition to the use of stents, various drugs have been applied to the site of the dilated lesion to prevent or reduce chances of restenosis and to aid in the healing of flaps, dissection or other hemorrhagic conditions that may appear after an angioplasty procedure. The prior art braided wire and balloon stents, as disclosed, for example, in U.S. Pat. Nos. 4,655,771, 5,295,962, 5,368,566 and 5,421,826, cannot be used to deliver or inject fluid-based agents to the specific site of the lesion while maintaining adequate flow in the vascular lumen. The fluid flow through the lumen is substantially blocked by these stents during use.
In recognition of this problem, temporary stenting catheters with drug delivery capabilities have been developed, as disclosed, for example, in U.S. Pat. Nos. 5,383,928 and 5,415,637. The '928 patent discloses a coil-shaped stent covered by a polymer sheath for local drug delivery. A drug is incorporated into the polymer sheath for controlled release of the drug upon insertion. Because the polymer sheath itself is as large as the diameter of the coil, the device cannot be removed from the subject outside of the lab and without a guiding catheter. Moreover, the device is limited in its ability to adapt to the shape and size of the vessel wall, and in that only drugs that are compatible with and can be incorporated into the polymer can be delivered by the device.
The temporary stenting catheter of the '637 patent functions to hold a collapsed dissected lining or flap against the blood vessel wall for a sufficient time to allow the natural adhesion of the flap to the blood vessel wall. The stenting catheter of the '637 patent also functions to introduce a drug to the site of the vascular procedure to aid in the adhesion process and in the prevention of restenosis while allowing the flow of blood through the vessel to locations distal to the catheter.
The catheter assembly of the '637 patent, however, has a number of disadvantages. The catheter assembly is complex and expensive to manufacture. More importantly, however, the catheter assembly of the '637 is very expensive to use because it requires a guiding catheter to be maintained within the vessel and the patient to be maintained within the catheter lab during use and deployment.
There are other known devices, as disclosed, for example, in U.S. Pat. Nos. 4,531,933, 4,694,838, 4,813,925, 4,887,996, and 5,163,928, that use a catheter having a heat set polymer stent at a distal end shaped as a halo or coil. These devices require pushing a rod through the lumen of the heat set curve in the polymer to straighten the device so that it may be inserted into the body through a guide catheter. The rod is then removed and the curve shape of the catheter comes back. These devices suffer from being limited in size due to the fact that a relatively large wire must be used to straighten the device (e.g., 0.014" to 0.016"). Thus, the lumen size of these devices are correspondingly large. The devices also suffer from a lack of ability to be deformed (i.e., coiled, bent, or shaped) without permanent deformation. Because of the permanent deformation, the devices fail to track the inside of a vessel that is not round.
The prior art drug delivery systems cannot be left in place for a period of time outside the lab in which the angioplasty was performed, and then removed by a nurse by simply pulling the system out. Any coil that relies on a balloon for its intravascular shape requires inflation to maintain the coil's shape, and the patient is required to stay in the lab under fluoro to make sure the coil stays in place. Any coil that relies on the modules of the polymer to maintain the coil shape is too rigid to pull out and requires the use of a straightening rod to push through the coil so that the coil straightens out before removal. This procedure would not be permitted outside the catheter lab, thus, adding significant cost to the procedure.
Another method of preventing or controlling restenosis following angioplasty has been developed in recent years which uses intravascular radiotherapy (IRT). IRT may also be used to prevent stenosis following cardiovascular graft procedures or other trauma to the vessel wall. IRT involves introducing a radioactive material, such as a beta emitting material (for example, .sup.32 P) or a photon emitting material (for example, .sup.125 I). into a blood vessel for a predetermined time to provide a radiation dosage. The radiation dosage must be carefully controlled to impair or arrest hyperplasia without causing excessive damage to healthy tissue. Overdosing of a section of blood vessel can cause arterial necrosis, inflammation and hemorrhaging, while under dosing will result in no inhibition of smooth muscle cell hyperplasia, or even exacerbation of the hyperplasia and resulting restenosis.
An IRT method using a radioactive stent is disclosed in U.S. Pat. No. 5,059,166. The radioactive stent is designed to be permanently implanted in the blood vessel after completion of a lumen opening procedure. The radiation dose delivered to the patient is determined by the activity of the stent at the particular time it is implanted.
Another IRT method is disclosed in U.S. Pat. No. 5,302,168, which uses a radioactive source contained in a flexible carrier with remotely manipulated windows. This method generally requires the use of a higher activity source than the radioactive stent to deliver an effective dose to the patient. Accordingly, measures must be taken to ensure that the source is maintained at the center of the lumen to prevent localized overexposure of tissue to the radiation source. Use of a higher activity source also requires expensive shielding and other equipment for safe handling. Conventional IRT methods also tend to block or restrict blood flow through the vessel during treatment.
Thus, there is a need for an improved system and method for delivering diagnostic and therapeutic agents intravascularly that overcomes the problems of the existing systems.